fix: format execution engine

[stefan-jansen]

Summary

• apply the missing formatter change for execution_engine.py from the previous main-CI repair

Validation

• uv run ruff format --check src tests
• uv run ruff check src tests
• uv run ty check

[Copilot]

Pull request overview

Applies the previously-missed formatting update to execution_engine.py, aligning it with the formatter output from the earlier main-CI repair.

Changes:

• Reformats the partial-fill eligibility elif block in _process_single_order for readability.
• Tightens the rebalance/integer partial-fill path to also require an “insufficient” rejection reason.

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[Copilot]

The mixed and/or condition here is hard to read and repeats "insufficient" in rejection_reason.lower() (and calls .lower() multiple times in this function). Consider computing a single insufficient_cash = "insufficient" in rejection_reason.lower() once and then expressing the condition as insufficient_cash AND (partial fills allowed OR (rebalance_id set AND share_type is integer)) to make precedence unambiguous and avoid duplicated work.